formula: mkpath bin before writing the launcher

Nothing creates <prefix>/bin in this formula — the payload goes to libexec —
and Homebrew's Pathname#write does not create parent directories. (Its own
write_env_script calls dirname.mkpath by hand for exactly this reason,
extend/pathname.rb:347.) Without this, install dies on Errno::ENOENT at the
very first user's `brew install`.

Pathname#install is unaffected: install_p mkpaths its destination
(extend/pathname.rb:487), so libexec/prefix/doc installs were already fine.

Author: speak-agent

Formula/mcpp-m.rb

@@ -60,6 +60,11 @@ def install
     # dir — `brew upgrade` would then silently drop every installed toolchain.
     # Pin the two env knobs the binary honors, deferring to values the user
     # already exported. Mirrors the Arch launcher (upstream scripts/aur).
+    #
+    # bin.mkpath first: nothing has created <prefix>/bin at this point (the
+    # payload went to libexec), and Homebrew's Pathname#write does not make
+    # parent directories — write_env_script mkpaths by hand for this reason.
+    bin.mkpath
     (bin/"mcpp").write <<~SH
       #!/bin/sh
       export MCPP_HOME="${MCPP_HOME:-$HOME/.mcpp}"